Silicon-germanium optical modulator structure for use with optical chips

ABSTRACT

The optical device includes a waveguide positioned on a base and a modulator positioned on the base. The modulator includes a ridge that includes Si1-xGex where x is greater than or equal to 0.4 and less than or equal to 0.8. The modulator is configured to guide a light signal through the modulator such that the light signal contacts the Si1-xGex. A local heater is configured to heat the modulator.

FIELD

The present invention relates to optical devices and particularly, tooptical modulators.

BACKGROUND

Satisfying IEEE 802.3 standard for optical communications up to 2 kmgenerally requires the ability to modulate light signals havingwavelengths around 1310 nm. Planar optical devices are one platform forimplementing these standards. These platforms often have single modewaveguides with dimensions on the order of several microns. However, acommercially viable electroabsorption modulator that has these waveguidedimensions and satisfies these standards has yet to be built on theseplatforms. For instance, some materials that can be used in theseplatforms have shown the ability to modulate the light signals but haveshown impractically low levels of absorption. As a result, there is aneed to an improved modulator.

SUMMARY

The optical device includes a waveguide positioned on a base and amodulator positioned on the base. The modulator includes a ridge of anelectro-absorption medium having a top side and a lateral side. Thelateral side is between the top side and the base and the top side has awidth. The waveguide is configured to guide a light signal through themodulator such that the light signal is guided through the ridge ofelectro-absorption medium. A heater is positioned over the lateral sideof the electro-absorption medium without being positioned over theentire width of the ridge.

Another embodiment of the optical device includes a waveguide positionedon a base and a modulator positioned on the base. The modulator includesa ridge of an electro-absorption medium. The waveguide is configured toguide a light signal through the modulator such that the light signal isguided through the ridge of the electro-absorption medium. A deviceridge includes the ridge of the electro-absorption medium. The deviceridge has lateral sides that extend upward from a lower inside corner toa top side. The laterals sides are between the top side and the base. Aheater is positioned over one of the lateral side of the device ridgeand extends down to the lower inside corner of the device ridge.

Another embodiment of the device includes a waveguide positioned on abase and a modulator positioned on the base. The modulator includes aridge that includes a quantum well structure that includes multiplelayers. The layers include one or more wells and two or more potentialbarriers arranged such that wells alternate with potential barriers. Insome instances, the ridge includes Si_(1-x)Ge_(x) where x is greaterthan or equal to 0.4 and less than or equal to 0.8. The modulator isconfigured to guide a light signal through the modulator such that thelight signal is guided through the Si_(1-x)Ge_(x). A local heater isconfigured to heat the modulator. In some instances, a device ridgeincludes the ridge. The device ridge has a top side and lateral sides.The lateral sides are between the top side and the base and the top sidehas a width. A heater can be positioned over one of the lateral sidesand the top side of the device ridge without being positioned over theentire width of the top side.

Another embodiment of the device includes a waveguide positioned on abase and a modulator positioned on the base. The modulator is aQuantum-Confined Stark Effect (QCSE) electro-absorption modulator thatincludes a ridge having a quantum well structure that includes multiplelayers. The layers include one or more wells and two or more potentialbarriers arranged such that wells alternate with potential barriers. Insome instances, the ridge includes Si_(1-x)Ge_(x) where x is greaterthan or equal to 0.4 and less than or equal to 0.8. The modulator isconfigured to guide a light signal through the modulator such that thelight signal contacts the Si_(1-X)Ge_(X).

A method of fabricating a modulator includes growing a seed layer on abase such that a surface of the seed layer has a root mean squareroughness less than 1 nm. The method can also include growing a quantumwell structure on the surface of the seed layer. The quantum wellstructure includes multiple layers. The layers include one or more wellsand two or more potential barriers arranged such that wells alternatewith potential barriers. In some instances, the ridge includesSi_(1-x)Ge_(x) where x is greater than or equal to 0.4 and less than orequal to 0.8. In one example, the portion of the quantum well structurethat contacts the surface is epitaxially grown in a chamber at apressure of 20 to 60 Torr and/or at a temperature of 500 to 900 degrees.

A method of fabricating a modulator includes growing a quantum wellstructure on a base. The quantum well structure includes Si_(1-x)Ge_(x)where x is greater than or equal to 0.4 and less than or equal to 0.8.The method also includes growing the quantum well structure at atemperature of 500 to 900 degrees. In some instances, the quantum wellstructure alternates wells with potential barriers where the wells areGe and the potential barriers the Si_(1-x)Ge_(x).

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A and FIG. 1B illustrates an optical device having a waveguidethat guides a light signal between a light source and a modulator. FIG.1A is a perspective view of the device.

FIG. 1B is a cross section of the device taken along the line labeled Bin FIG. 1A.

FIG. 2A through FIG. 2E illustrate construction of a modulator that issuitable for use as the modulator of FIG. 1A. FIG. 2A is a topview ofthe portion of the optical device shown in FIG. 1A that includes theoptical modulator.

FIG. 2B is a cross-section of the optical device shown in FIG. 2A takenalong the line labeled B.

FIG. 2C is a cross-section of the optical device shown in FIG. 2A takenalong the line labeled C.

FIG. 2D is a cross-section of the optical device shown in FIG. 2A takenalong the line labeled D.

FIG. 2E is a cross-section of the optical device shown in FIG. 2A takenalong the line labeled E.

FIG. 3 is a cross section of an embodiment of a modulator having areduced sensitivity to the thickness of the slab regions on opposingsides of a waveguide.

FIG. 4A through FIG. 4C illustrates examples of the modulator of FIG. 2Emodified to include a quantum well structure located between layers ofthe electro-absorption medium. FIG. 4A is a cross-section of the portionof the device that includes a modulator.

FIG. 4B is a cross-section of the portion of the device that includesanother embodiment of a modulator.

FIG. 4C is a cross section of a portion of the quantum well structure inthe device of FIG. 4A and/or FIG. 4B.

FIG. 5A through FIG. 5D illustrate the waveguide in the device of FIG.2A through FIG. 2D modified to include transition portions. FIG. 5A is atopview of a portion of an optical device that includes an opticalmodulator.

FIG. 5B is a cross-section of the optical device shown in FIG. 5A takenalong the line labeled B.

FIG. 5C is a cross-section of the optical device shown in FIG. 5A takenalong the line labeled C.

FIG. 5D is a cross-section of the optical device shown in FIG. 5A takenalong the line labeled D.

FIG. 6A through FIG. 6C illustrate a localized heater in conjunctionwith a modulator. FIG. 6A is a topview of the portion of the device thatincludes the modulator.

FIG. 6B is a cross section of the modulator shown in FIG. 6A taken alongthe line labeled B in FIG. 6A.

FIG. 6C is a cross section of the modulator shown in FIG. 6A taken alongthe line labeled C in FIG. 6A.

FIG. 7A is a cross section of a portion of a device that includes aheater on a modulator. The heater is positioned over a lateral side ofthe modulator and over a portion of a device ridge.

FIG. 7B is a cross section of a portion of a device that includes aheater on a modulator. The heater is positioned over a lateral side ofthe modulator.

FIG. 7C is a cross section of a portion of a device that includes aheater on a modulator. The heater is positioned over a lateral side ofthe modulator, extends away from the lateral side, and is over a portionof a device ridge.

FIG. 8A and FIG. 8B illustrate a device having a modulator that includesheaters positioned over opposing lateral sides of a ridge.

FIG. 8B is a cross section of the device shown in FIG. 8A taken alongthe line labeled B in FIG. 8A.

FIG. 9A and FIG. 9B illustrate the device of FIG. 6A through FIG. 6C incombination with the modulator of FIG. 2E. FIG. 9A is a topview of thedevice.

FIG. 9B is a cross section of the device shown in FIG. 9A taken alongthe line labeled B in FIG. 9A

FIG. 10A and FIG. 10B illustrate the device of FIG. 9A and FIG. 9Bmodified so as to include thermal conductors. FIG. 10A is a topview ofthe device.

FIG. 10B is a cross section of the device shown in FIG. 10A taken alongthe line labeled B in FIG. 10A.

FIG. 11 shows the device of FIG. 2A modified such that an angle betweena modulator portion of a waveguide and a first portion of the waveguideis less than 180° and an angle between the modulator portion of thewaveguide and a second portion of the waveguide is less than 180°.

FIG. 12A is a cross section of a device precursor for use in fabricatinga heater.

FIG. 12B illustrates the device precursor of FIG. 12A after a firstetch.

FIG. 12C illustrates the device precursor of FIG. 12B after a secondetch.

FIG. 12D illustrates the device precursor of FIG. 12C after removal of amask from the device precursor of FIG. 12C.

FIG. 12E illustrates the device precursor of FIG. 12B after removal of afirst mask and the formation of a second mask.

DESCRIPTION

A device includes a waveguide positioned on a base and a modulatorpositioned on the base. The modulator is a Quantum-Confined Stark Effect(QCSE) electro-absorption modulator. The modulator includes a quantumwell structure that includes multiple layers. The layers include one ormore wells and two or more potential barriers arranged such that wellsalternate with potential barriers. In some instances, the potentialbarriers includes Si_(1-x)Ge_(x) where x is greater than or equal to 0.4and/or less than or equal to 0.8. The modulator is configured to guide alight signal through the modulator such that the light signal contactsthe Si_(1-x)Ge_(x). The modulator provides higher levels of absorptionof 1310 nm light than was achieved with bulk materials. As a result, themodulator is a commercially viable modulator of 1310 nm light that canbe constructed on planar optical device platforms.

It was previously believed that the disclosed structure could not bebuilt because the silicon and germanium in the different layers of thequantum well structure would diffuse as a result of the growth and/orannealing the device (non Srantski-Krastanov islanding). This diffusionwould cause the quantum well structure to behave as a bulksemiconductor. Since these bulk semiconductors have unacceptably lowlevels of absorption, it was believed these structures would result in amodulator that would also have low absorption levels. However, theinventors have surprisingly found that the growth conditions can betuned so as to achieve the desired results without diffusion of thesilicon and germanium.

FIG. 1A and FIG. 1B illustrate an optical device having a waveguide thatguides a light signal between a light source 8 and a modulator 9. FIG.1A is a perspective view of the device. FIG. 1B is a cross section ofthe device taken along the line labeled B in FIG. 1A. FIG. 1A and FIG.1B do not show details of either the light source 8 or the modulator butillustrates the relationship between these components and the waveguide.

The device is within the class of optical devices known as planaroptical devices. These devices typically include one or more waveguidesimmobilized relative to a substrate or a base. The direction ofpropagation of light signals along the waveguides is generally parallelto a plane of the device. Examples of the plane of the device includethe top side of the base, the bottom side of the base, the top side ofthe substrate, and/or the bottom side of the substrate.

The illustrated device includes lateral sides 10 (or edges) extendingfrom a top side 12 to a bottom side 14. The propagation direction oflight signals along the length of the waveguides on a planar opticaldevice generally extends through the lateral sides 10 of the device. Thetop side 12 and the bottom side 14 of the device are non-lateral sides.

The device includes one or more waveguides 16 that carry light signalsto and/or from optical components 17. Examples of optical components 17that can be included on the device include, but are not limited to, oneor more components selected from a group consisting of facets throughwhich light signals can enter and/or exit a waveguide, entry/exit portsthrough which light signals can enter and/or exit a waveguide from aboveor below the device, multiplexers for combining multiple light signalsonto a single waveguide, demultiplexers for separating multiple lightsignals such that different light signals are received on differentwaveguides, optical couplers, optical switches, lasers that act as asource of a light signal, amplifiers for amplifying the intensity of alight signal, attenuators for attenuating the intensity of a lightsignal, modulators for modulating a signal onto a light signal,modulators that convert a light signal to an electrical signal, and viasthat provide an optical pathway for a light signal traveling through thedevice from the bottom side 14 of the device to the top side 12 of thedevice. Additionally, the device can optionally, include electricalcomponents. For instance, the device can include electrical connectionsfor applying a potential or current to a waveguide and/or forcontrolling other components on the optical device.

A portion of the waveguide includes a first structure where a portion ofthe waveguide 16 is defined in a light-transmitting medium 18 positionedon a base 20. For instance, a portion of the waveguide 16 is partiallydefined by a ridge 22 extending upward from a slab region of thelight-transmitting medium as shown in FIG. 1B. In some instances, thetop of the slab region is defined by the bottom of trenches 24 extendingpartially into the light-transmitting medium 18 or through thelight-transmitting medium 18. Suitable light-transmitting media include,but are not limited to, silicon, polymers, silica, SiN, GaAs, InP andLiNbO₃.

The portion of the base 20 adjacent to the light-transmitting medium 18is configured to reflect light signals from the waveguide 16 back intothe waveguide 16 in order to constrain light signals in the waveguide16. For instance, the portion of the base 20 adjacent to thelight-transmitting medium 18 can be a light insulator 28 with a lowerindex of refraction than the light-transmitting medium 18. The drop inthe index of refraction can cause reflection of a light signal from thelight-transmitting medium 18 back into the light-transmitting medium 18.The base 20 can include the light insulator 28 positioned on a substrate29. As will become evident below, the substrate 29 can be configured totransmit light signals. For instance, the substrate 29 can beconstructed of a light-transmitting medium 18 that is different from thelight-transmitting medium 18 or the same as the light-transmittingmedium 18. In one example, the device is constructed on asilicon-on-insulator wafer. A silicon-on-insulator wafer includes asilicon layer that serves as the light-transmitting medium 18. Thesilicon-on-insulator wafer also includes a layer of silica positioned ona silicon substrate. The layer of silica can serving as the lightinsulator 28 and the silicon substrate can serve as the substrate 29.

Although the light source 8 is shown positioned centrally on the device,the light source 8 can be positioned at the edge of the device. Thelight source 8 can be any type of light source including light sourcesthat convert electrical energy into light. Examples of suitable lightsources include, but are not limited to, a semiconductor laser, and asemiconductor amplifier such as a reflection semiconducting opticalamplifier (RSOA) forming a laser cavity with an external reflectiongrating. Examples of suitable lasers include, but are not limited to,Fabry-Perot lasers, Distributed Bragg Reflector lasers (DBR lasers),Distributed FeedBack lasers (DFB lasers), external cavity lasers (ECLs).A variety of suitable lasers and laser constructions are disclosed inlight source applications including U.S. patent application Ser. No.13/385,774, filed on Mar. 5, 2012, and entitled “Integration ofComponents on Optical Device;” U.S. patent application Ser. No.14/048,685, filed on Oct. 8, 2013, and entitled “Use of Common ActiveMaterials in Optical Components;” U.S. Provisional Patent ApplicationSer. No. 61/825,501, filed on May 20, 2013, and entitled “Reducing PowerRequirements for Optical Links;” U.S. patent application Ser. No.13/694,047, filed on Oct. 22, 2012, and entitled “Wafer Level Testing ofOptical Components;” U.S. patent application Ser. No. 13/506,629, filedon May 2, 2012, and entitled “Integration of Laser into OpticalPlatform;” U.S. patent application Ser. No. 13/573,892, filed on Oct.12, 2012, and entitled “Reduction of Mode Hopping in a Laser Cavity;”U.S. patent application Ser. No. 13/317,340, filed on Oct. 14, 2011, andentitled “Gain Medium Providing Laser and Amplifier Functionality toOptical Device;” U.S. patent application Ser. No. 13/385,275, filed onFeb. 9, 2012, and entitled “Laser Combining Light Signals from MultipleLaser Cavities;” each of which is incorporated herein in its entirety.The light source 8 can be constructed as disclosed in any one or more ofthe light source applications and/or can be interfaced with the deviceas disclosed in any one or more of the light source applications. Othersuitable light sources include interdevice waveguides that carry a lightsignal to the device from another device such as an optical fiber. Avariety of interfaces between an optical fiber and a device constructedaccording to FIG. 1A and FIG. 1B are disclosed in fiber interfacepatents applications including U.S. patent application Ser. No.12/228,007, filed on Nov. 14, 2008, and entitled “Optical System HavingOptical Fiber Mounted to Optical Device,” now abandoned; and U.S. patentapplication Ser. No. 12/148,784, filed on Apr. 21, 2008, entitled“Transfer of Light Signals Between Optical Fiber and System UsingOptical Devices with Optical Vias,” and issued as U.S. Pat. No.8,090,231; each of which is incorporated herein in its entirety. Thelight source 8 can an optical fiber interfaced with a device asdisclosed in any one or more of the fiber interface patentsapplications. In some instances, the device does not include a lightsource. For instance, the waveguide can terminate at a facet located ator near the perimeter of the device and a light signal traveling throughair can then be injected into the waveguide through the facet.Accordingly, the light source is optional.

FIG. 2A through FIG. 2E illustrate construction of a modulator that issuitable for use as the modulator of FIG. 1A. FIG. 2A is a topview ofthe portion of the optical device shown in FIG. 2A that includes anoptical modulator. FIG. 2B is a cross-section of the optical deviceshown in FIG. 2A taken along the line labeled B. FIG. 2C is across-section of the optical device shown in FIG. 2A taken along theline labeled C. FIG. 2D is a cross-section of the optical device shownin FIG. 2A taken along the line labeled D. FIG. 2E is a cross-section ofthe optical device shown in FIG. 2A taken along the line labeled E.

Recesses 25 (FIG. 2A) extend into the slab regions such that the ridge22 is positioned between recesses 25. The recesses 25 can extend partway into the light-transmitting medium 18. As is evident from FIG. 2B,the recesses 25 can be spaced apart from the ridge 22. As a result, aportion of the waveguide 16 includes a second structure where an upperportion of the waveguide 16 is partially defined by the ridge 22extending upward from the slab region and a lower portion of thewaveguide is partially defined by recesses 25 extending into the slabregions and spaced apart from the ridge.

As shown in FIG. 2C, the recesses 25 can approach the ridge 22 such thatthe sides of the ridge 22 and the sides of the recesses 25 combine intoa single surface 26. As a result, a portion of a waveguide includes athird structure where the waveguide is partially defined by the surface26.

As is evident in FIG. 2A, a portion of the waveguide 16 includes anelectro-absorption medium 27. The electro-absorption medium 27 isconfigured to receive the light signals from a portion of the waveguidehaving the third structure and to guide the received light signals toanother portion of the waveguide having the third structure.

In FIG. 2D, a ridge 22 of electro-absorption medium 27 extends upwardfrom a slab region of the electro-absorption medium 27. Accordingly, aportion of a waveguide includes a fourth structure configured to guidethe received light signal through the electro-absorption medium 27. Thisportion of the waveguide is partially defined by the top and lateralsides of the electro-absorption medium 27. The slab regions of theelectro-absorption medium 27 and the ridge 22 of the electro-absorptionmedium 27 are both positioned on a seed portion 34 of thelight-transmitting medium 18. As a result, the seed portion 34 of thelight-transmitting medium 18 is between the electro-absorption medium 27and the base 20. In some instances, when the light signal travels fromthe light-transmitting medium into the electro-absorption medium 27, aportion of the light signal enters the seed portion 34 of thelight-transmitting medium 18 and another portion of the light signalenters the electro-absorption medium 27. As described above, theelectro-absorption medium 27 can be grown on the seed portion of thelight-transmitting medium 18. The seed layer is optional. For instance,the electro-absorption medium 27 can be grown or otherwise formeddirectly on the seed portion of the light-transmitting medium 18

As is evident in FIG. 2A, there is an interface between each facet ofthe electro-absorption medium 27 and a facet of the light-transmittingmedium 18. The interface can have an angle that is non-perpendicularrelative to the direction of propagation of light signals through thewaveguide 16 at the interface. In some instances, the interface issubstantially perpendicular relative to the base 20 while beingnon-perpendicular relative to the direction of propagation. Thenon-perpendicularity of the interface reduces the effects of backreflection. Suitable angles for the interface relative to the directionof propagation include but are not limited to, angles between 80° and89°, and angles between 80° and 85°.

The optical device includes a modulator. The location of the modulatoron the optical device is illustrated by the line labeled K in FIG. 2A.In order to simplify FIG. 2A, the details of the modulator constructionare not shown in FIG. 2A. However, the modulator construction is evidentfrom other illustrations such as FIG. 2E. The modulator of FIG. 2E isconstructed on the portion of the waveguide having a fourth structureconstructed according to FIG. 2D. The perimeter of portions of dopedregions shown in FIG. 2E are illustrated with dashed lines to preventthem from being confused with interfaces between different materials.The interfaces between different materials are illustrated with solidlines. The modulator is configured to apply an electric field to theelectro-absorption medium 27 in order to phase and/or intensity modulatethe light signals received by the modulator.

A ridge 22 of the electro-absorption medium 27 extends upward from aslab region of the electro-absorption medium 27. Doped regions 40 areboth in the slab regions of the electro-absorption medium 27 and also inthe ridge of the electro-absorption medium 27. For instance, dopedregions 40 of the electro-absorption medium 27 are positioned on thelateral sides of the ridge 22 of the electro-absorption medium 27. Insome instances, each of the doped regions 40 extends up to the top sideof the electro-absorption medium 27 as shown in FIG. 2E. Additionally,the doped regions 40 extend away from the ridge 22 into the slab regionof the electro-absorption medium 27. The transition of a doped region 40from the ridge 22 of the electro-absorption medium 27 into the slabregion of the electro-absorption medium 27 can be continuous andunbroken as shown in FIG. 2E.

Each of the doped regions 40 can be an N-type doped region or a P-typedoped region. For instance, each of the N-type doped regions can includean N-type dopant and each of the P-type doped regions can include aP-type dopant. In some instances, the electro-absorption medium 27includes a doped region 40 that is an N-type doped region and a dopedregion 40 that is a P-type doped region. The separation between thedoped regions 40 in the electro-absorption medium 27 results in theformation of PIN (p-type region-insulator-n-type region) junction in themodulator.

In the electro-absorption medium 27, suitable dopants for N-type regionsinclude, but are not limited to, phosphorus and/or arsenic. Suitabledopants for P-type regions include, but are not limited to, boron. Thedoped regions 40 are doped so as to be electrically conducting. Asuitable concentration for the P-type dopant in a P-type doped regionincludes, but is not limited to, concentrations greater than 1×10¹⁵cm⁻³, 1×10¹⁷ cm⁻³, or 1×10¹⁹ cm³, and/or less than 1×10¹⁷ cm⁻³, 1×10¹⁹cm⁻³, or 1×10²¹ cm⁻³. A suitable concentration for the N-type dopant inan N-type doped region includes, but is not limited to, concentrationsgreater than 1×10¹⁵ cm⁻³, 1×10¹⁷ cm⁻³, or 1×10¹⁹ cm⁻³, and/or less than1×10¹⁷ cm⁻³, 1×10¹⁹ cm⁻³, or 1×10²¹ cm⁻³.

Electrical conductors 44 each are positioned on one of the slab regionsof the electro-absorption medium 27. In particular, each electricalconductor 44 contacts a portion of a doped region 40 that is in the slabregion of the electro-absorption medium 27. Accordingly, each of thedoped regions 40 is doped at a concentration that allows it to provideelectrical communication between an electrical conductor 44 and one ofthe doped regions 40 in the electro-absorption medium 27. As a result,electrical energy can be applied to the electrical conductors 44 inorder to apply an electric field to the electro-absorption medium 27.The region of the light-transmitting medium or electro-absorption mediumbetween the doped regions can be undoped or lightly doped as long as thedoping is insufficient for the doped material to act as an electricalconductor that electrically shorts the modulator.

During operation of the modulators of FIG. 1A through FIG. 2E,electronics 47 (FIG. 1A) can be employed to apply electrical energy tothe electrical conductors 44 so as to form an electrical field in theelectro-absorption medium 27. For instance, the electronics can form avoltage differential between the doped regions that act as a source ofthe electrical field in the gain medium. The electrical field can beformed without generating a significant electrical current through theelectro-absorption medium 27. The electro-absorption medium 27 can be amedium in which the Franz-Keldysh effect occurs in response to theapplication of the electrical field. The Franz-Keldysh effect is achange in optical absorption and optical phase by an electro-absorptionmedium 27. For instance, the Franz-Keldysh effect allows an electron ina valence band to be excited into a conduction band by absorbing aphoton even though the energy of the photon is below the band gap. Toutilize the Franz-Keldysh effect the active region can have a slightlylarger bandgap energy than the photon energy of the light to bemodulated. The application of the field lowers the absorption edge viathe Franz-Keldysh effect and makes absorption possible. The hole andelectron carrier wavefunctions overlap once the field is applied andthus generation of an electron-hole pair is made possible. As a result,the electro-absorption medium 27 can absorb light signals received bythe electro-absorption medium 27 and increasing the electrical fieldincreases the amount of light absorbed by the electro-absorption medium27. Accordingly, the electronics can tune the electrical field so as totune the amount of light absorbed by the electro-ab sorption medium 27.As a result, the electronics can intensity modulate the electrical fieldin order to modulate the light signal. Additionally, the electricalfield needed to take advantage of the Franz-Keldysh effect generallydoes not involve generation of free carriers by the electric field.

A modulator having a cross section according to FIG. 2E can be sensitiveto the thickness of the slab regions of the electro-absorption medium27. For instance, as the thickness of the slab region increases, theridge becomes smaller and the electrical field formed between the dopedregions 40 accordingly fills a smaller portion of the distance betweenthe base 20 and the top of the ridge. For instance, the location of theelectrical field effectively moves upwards from the base 20. Theincreased space between the electrical field and the base 20 can bethought of as increasing the resistance or carrier diffusion time of themodulator. This increase in resistance and/or diffusion time decreasesthe speed of the modulator. Problems also occur when these slab regionsbecome undesirably thin. When these slab regions become thin, the dopedregions extend down into the light-transmitting medium 18. This dopingof the light-transmitting medium 18 also decreases the speed of themodulator.

FIG. 3 presents an embodiment of a modulator having a reducedsensitivity to the thickness of the slab regions. The perimeter ofportions of doped regions shown in FIG. 3 are illustrated with dashedlines to prevent them from being confused with interfaces betweendifferent materials. The interfaces between different materials areillustrated with solid lines. A first doped zone 45 and a second dopedzone 46 combine to form each of the doped regions 40. In some instance,the first doped zone 45 is located in the light-transmitting medium 18but not in the electro-absorption medium 27 and the second doped zone 46is located in the electro-absorption medium 27. The first doped zone 45can contact the second doped zone 46 or can overlap with the seconddoped zone 46. In some instances, the first doped zone 45 and the seconddoped zone 46 overlap and at least a portion of the overlap is locatedin the light-transmitting medium 18. In other instances, the first dopedzone 45 and the second doped zone 46 overlap without any overlap beingpresent in the electro-absorption medium 27.

The first doped zone 45 and the second doped zone 46 included in thesame doped region 40 each includes the same type of dopant. Forinstance, the first doped zone 45 and the second doped zone 46 in ann-type doped region 40 each includes an n-type dopant. The first dopedzone 45 and the second doped zone 46 included in the same doped region40 can have the same dopant concentration or different concentrations.

Although FIG. 3 illustrates the slab regions including theelectro-absorption medium 27, the slab regions of the electro-absorptionmedium 27 may not be present. For instance, the etch that forms the slabregions of the electro-absorption medium 27 may etch all the way throughthe slab regions. In these instances, the first doped zone 45 and thesecond doped zone 46 are both formed in the light-transmitting medium.

Although FIG. 3 shows the first doped zone 45 not extending down to thelight insulator 28, the first doped zone 45 can extend down to the lightinsulator 28 or into the light insulator 28.

Suitable electro-absorption media 27 for use in the modulator includesemiconductors. However, the light absorption characteristics ofdifferent semiconductors are different. A suitable semiconductor for usewith modulators employed in communications applications includesGe_(1-x)Si_(x) (germanium-silicon) where x is greater than or equal tozero. In some instances, x is less than 0.05, or 0.01. Changing thevariable x can shift the range of wavelengths at which modulation ismost efficient. For instance, when x is zero, the modulator is suitablefor a range of 1610-1640 nm. Increasing the value of x can shift therange of wavelengths to lower values. For instance, an x of about 0.005to 0.01 is suitable for modulating in the c-band (1530-1565 nm).

Another suitable semiconductor for use as the electro-absorption media27 in a modulator for communications applications includesSi_(1-x)Ge_(x) where x is greater than or equal to 0.4 and less than orequal to 0.8. These modulators are suitable for modulating wavelengthsin a range of 1200 nm to 1600 nm and more particularly, 1250 nm to 1350nm. Accordingly, these modulators can satisfy the IEEE specification foroptical communications up to 2 km. Changing the variable x can shift therange of wavelengths and the absorption at which modulation is mostefficient. For instance, when x is 0.6, the modulator is suitable for arange of 1250-1350 nm. Increasing the value of x can shift the range ofwavelengths to higher values.

When the above modulators are constructed using Si_(1-x)Ge_(x)(0.4≤x≤0.8) so as to modulate wavelengths in a range 1200 nm to 1600 nm,the level of absorption may be undesirably low. The inventors have foundthat the low absorption levels can be overcome by constructing themodulator to include a quantum well structure constructed such that themodulator operates as a Quantum-Confined Stark Effect (QCSE)electro-absorption modulator.

FIG. 4A and FIG. 4B illustrates examples of the modulator of FIG. 2Emodified to include a Quantum Well structure 31 located between layersof the electro-absorption medium 27. The modulator of FIG. 4A includesthe doped regions 40 positioned above and below the Quantum Wellstructure 31. The modulator of FIG. 4B includes the doped regionspositioned on the lateral sides of the Quantum Well structure 31. FIG.4C is a magnified view of a portion of the Quantum Well structure 31 ofFIG. 4A and/or FIG. 4B.

The modulators of FIG. 4A and FIG. 4B are configured such that themodulator guides the light signals through the quantum well structure31. The quantum well structure 31 includes multiple layers 47 as shownin FIG. 4A through FIG. 4C. The layers can include or consist of one ormore sub-layers. For instance, the layers 47 in the quantum wellstructure of FIG. 4C are illustrated as including four sub-layers 48.The sub-layers 48 and the layers 47 are arranged such that the quantumwell structure includes potential barriers 49 that alternate withquantum wells 50. In one example, the quantum well structure includesSi_(1-x)Ge_(x) (0.4≤x≤0.8). For instance, the quantum well can be suchthat Si_(1-x)Ge_(x) (0.4≤x≤0.8) serves as the potential barrier andGe_(y)Si_(x) where 0.9≤y≤0.99 and 0.1≤x≤0.01 or 0.9≤y≤1.0 and 0.1≤x≤0.0serves as the quantum wells. Other suitable materials for the potentialbarriers include, but are not limited to, SiSn, SiGe, and GeSn.

In some instances, all or a portion of the quantum well structure isconfigured to have a periodic structure. For instance, the layers 47 caneach have the same chemical composition. When the layers include morethan one sub-layer 48, corresponding sub-layers in different layers 47can each have the same composition. For instance, each of the quantumwells 50 can be Si_(0.4)Ge_(0.6) and each of the potential barriers 49can be Ge.

The number of layers 47 and/or the number sub-layers 48 in each layer 47in the quantum well structure can be changed in order to tune theabsorption and/or Extinction Ratio (ER) of the modulator. In someinstances, the number sub-layers 48 in each layer 47 is greater than isgreater than 2 and less then 6, and/or the number of layers 47 in thequantum well structure is greater than 6, 7, or 8 and less than 30, 40,or 100. As the number of quantum wells 50 increases, the modulatorstructure of FIG. 4A may experience increased capacitance effects. As aresult, the modulator structure of FIG. 4B may provide higher rates withincreasing numbers of quantum wells 50.

The thickness of the wells 50 is labeled Tw and the thickness of thepotential barriers 49 is labeled Tb in FIG. 4C. The thickness of thewells 50 can be the same or different from the thickness of thepotential barriers 49. In some instances, the thickness of the wells 50is greater than 2 nm, 6 nm, or 7 nm and less then 11 nm, 12 nm, or 30 nmand the thickness of the potential barriers 49 is greater than 2 nm, 9nm, or 10 nm and/or less than 20 nm, 30 nm, or 100 nm. In one example,the thickness of the wells 50 is 9 nm and the thickness of the potentialbarriers 49 is 12 nm.

The thickness of the upper layer of electro-absorption medium 27 islabeled Tu and thickness of the lower layer of electro-absorption medium27 is labeled Tl in FIG. 4A and FIG. 4B. The thickness for the upperlayer of electro-absorption medium 27 (Tu) can be the same or differentfrom the thickness for the lower layer of electro-absorption medium 27(Tu). In some instances, the quantum well structure fills the entireridge of the electro-absorption medium 27. Accordingly, the thicknessfor the upper layer of electro-absorption medium 27 (Tu) can be 0.0and/or the thickness for the upper layer of electro-absorption medium 27(Tu) can be 0.0. In some instances, a thickness for the upper layer ofelectro-absorption medium 27 (Tu) is greater than 1 μm and less than 1.5μm and/or a thickness for the lower layer of electro-absorption medium27 (Tl) is greater than 0.9 μm and less than 1.3 μm.

The inventors have found that the Quantum-Confined Stark Effect (QCSE)modulators constructed disclosed above can include a localized heaterconfigured to heat all or a portion of the modulator. The temperaturechange can be used to tune the working wavelength of the modulator. Theinventors have found that the Quantum-Confined Stark Effect (QCSE)modulators constructed as disclosed above can have a working wavelengthshift rate of greater than 0.7 nm/° C.

The potential barriers 49 and the wells 50 can be formed by epitaxialgrowth processes such as a fast epitaxial growth process. In someinstances, the upper layer of the electro-absorption medium 27 is alsoformed by epitaxial growth. However, the quality of the absorption (peakto value ratio) of the modulator is a function of the smoothness of theupper and lower surfaces of the potential barriers 49 and the wells 50.When the potential barriers 49 and the wells 50 are formed by a seriesof epitaxial growth processes, the smoothness of the surface of the seedlayer upon which the first potential barrier 49 or well 50 (the uppersurface of the lower layer of the electro-absorption medium 27 in FIG.4A and/or FIG. 4B) is grown can affect the Extinction Ratio, the totalabsorption level, and the ratio of the absorption level:ExtinctionRatio. The inventors have found that the smoothness of the upper surfaceof the seed layer is a strong function of the chamber pressure duringepitaxial growth of the seed layer. For instance, the inventors havefound that when the upper surface of the lower layer of theelectro-absorption medium 27 is Si_(0.3)Ge_(0.7) epitaxially grown at apressure of 30, 40, and 50 Torr to a thickness of 300 nm, the root meansquared roughness is respectively 1.067, 0.953, and 0.82. Accordingly,the upper surface of the seed layer can have a root mean squareroughness greater than 1.5 nm and less than 3 nm. The inventors havefound that when the potential barriers 49 and wells 50 are epitaxiallygrown on the seed layer, the smoothness of the seed layer is carried oris essentially carried through the potential barriers 49 and wells 50 ofthe quantum well structure. For instance, the inventors have found thatthe root mean squared roughness of the potential barriers 49 and wells50 increases as thickness is added to the quantum well structure. Asuitable chamber pressure for growth of the seed layer, barriers 49 andwells 50 is greater than 10 Torr, or 20 Torr and/or less than 60 Torr,or 70 Torr. For instance, all or a portion of the wells 50 and potentialbarriers 49 can each be grown in a chamber at a pressure greater than 10Torr, or 20 Torr and/or less than 60 Torr, or 70 Torr.

The above structure has not been previously created due to the beliefthat the diffusion of the Si and Ge in the quantum well structure inresponse to growth conditions would cause the quantum well structure tobehave as the bulk structure with low levels of absorption. However, theinventors have surprisingly found that growth of the wells 50 andpotential barriers 49 can be done such that the structure to retain theSi and Ge within the wells 50 and potential barriers 49. Suitabletemperatures for growth of the quantum well structure include,temperatures greater than 300° C., or 400° C. and less than 700° C., or800° C. For instance, all or a portion of the wells 50 and potentialbarriers 49 can each be grown in a chamber at temperatures greater than300° C., or 400° C. and/or less than 700° C., or 800° C. As a result,the quantum well structure can be grown in a in a chamber at a pressuregreater than 10 Torr, or 20 Torr and/or less than 60 Torr, or 70 Torrand a temperature greater than 300° C., or 400° C. and less than 700°C., or 800° C. Accordingly, all or a portion of the wells 50 andpotential barriers 49 can each be grown in a chamber at a pressuregreater than 10 Torr, or 20 Torr and/or less than 60 Torr, or 70 Torrand a temperature greater than 300° C., or 400° C. and less than 700°C., or 800° C. When these temperatures and/or pressures are used duringthe growth of a layer selected from a well 50 or potential barrier 49,the temperature and/or pressure in the chamber can be maintained duringthe growth of all or a portion of the layer.

The growth of the quantum well structure can be followed by annealing ofthe quantum well structure. The annealing can be done in one annealingperiod or can be done in two, three or more than three annealing periodsseparated by rest periods. During the annealing periods, the quantumwell structure can be exposed to pressures greater 10 Torr, or 20 Torrand less than 60 Torr, or 70 Torr and/or temperatures greater than 600°C. or 700° C. and/or less than 800° C. or 900° C. for time periodsgreater than 10 min or 12 min and/or less than 18 min or 20 min. Duringthe rest periods, the quantum well structure is exposed to ultra-highvacuum and/or temperatures greater than 300° C. or 400° C. and/or lessthan 700° C. or 800° C. for time periods greater than 1 min or 2 minand/or less than 14 min or 15 min.

The waveguide can include one or more transition portions 53. Forinstance, FIG. 5A through FIG. 5D illustrate the waveguide in the deviceof FIG. 2A through FIG. 2D modified to include transition portions 53.FIG. 5A is a topview of the portion of an optical device that includesan optical modulator. FIG. 5B is a cross-section of the optical deviceshown in FIG. 5A taken along the line labeled B. FIG. 5C is across-section of the optical device shown in FIG. 5A taken along theline labeled C. FIG. 5D is a cross-section of the optical device shownin FIG. 5A taken along the line labeled D.

The device is modified such that the portions of the waveguide that havethe third structure and the fourth structure include theelectro-absorption media 27 in the ridge. For instance, the ridge 22 inFIG. 5C and FIG. 5D each includes the electro-absorption media 27.Although the electro-absorption medium 27 is shown as being positionedover the light-transmitting medium 18, the electro-absorption medium 27can be in contact with the base. For instance, the transition portion 53of the waveguide need not have light-transmitting medium 18 between theelectro-absorption medium 27 and the base 20. The electro-absorptionmedium 27 included in the transition portions 53 can be continuous withthe electro-absorption medium 27 included in the modulator.

FIG. 5A through FIG. 5D include the modulator located between thetransition portions 53. The presence of the transition portions 53 canreduce reflections caused by the quantum well structure 31. While FIG.5a through FIG. 5d illustrate the waveguide including multipletransition portions 53, the device can include a single transitionportion.

A suitable length that a transition region extends away from themodulator includes, but is not limited to, a length greater than 50 μm,80 μm, or 100 μm and/or less than 500 μm, 800 μm, or 1000 μm.

The location of a Quantum Well structure 31 shown in FIG. 5A. However,the modulator on the device of FIG. 5A through FIG. 5D can have any ofthe modulator structures disclosed above. Accordingly, the modulator onthe device of FIG. 5A through FIG. 5D need not have a Quantum Wellstructure 31.

Although FIG. 5A through FIG. 5D illustrate the electro-absorptionmedium 27 included in the portion of the waveguide having the thirdstructure and the fourth structure, the waveguide can have otherconfigurations. For instance, the portion of the waveguide having thefourth structure can include the electro-absorption medium 27 while theportion of the waveguide having the third structure excludes theelectro-absorption medium 27. Alternately, the electro-absorption medium27 can be included in the portion of the waveguide having the secondstructure, the third structure and the fourth structure. Alternately,the electro-absorption medium 27 can be included in the portion of thewaveguide having the first structure, the second structure, the thirdstructure and the fourth structure.

The localized heaters are not illustrated in FIG. 2A through FIG. 4C inorder to illustrate the parts located under the heater. However, FIG. 6Athrough FIG. 6C illustrate the localized heater in conjunction with amodulator. The details of the modulator are not illustrated, but themodulator can be constructed according to FIG. 2E through FIG. 4C or canhave another construction. FIG. 6A is a topview of the portion of thedevice that includes the modulator. FIG. 6B is a cross section of themodulator shown in FIG. 6A taken along the line labeled B in FIG. 6A.FIG. 6C is a cross section of the modulator shown in FIG. 6A taken alongthe longitudinal axis of the waveguide 16.

The heater 51 is on a lateral side the ridge 22 without extending over atop of the ridge 22. For instance, the heater 51 is positioned such thatan imaginary line can be drawn perpendicular to a lateral side of theridge 22 and extending through the heater 51. One or more layers ofmaterial can optionally be positioned between the heater 51 and theridge 22. For instance, the heater 51 can be located on an insulatinglayer 52 that electrically insulates the heater from the underlyinglayers. The insulating layer 52 is positioned between the heater and theridge 22. Suitable insulating layers 52 include, but are not limited to,silica and silicon nitride. An insulating layer with a higher thermalconductivity may be preferred in or to provide a pathway from heat totravel from the heater to the modulator. Accordingly, insulating layers52 that are thinner and/or have a higher thermal conductivity may bedesired. In some instances, the insulating layer 52 has a thermalconductivity above 10 W/mK.

One or more claddings 54 are optionally positioned between the waveguide16 and the insulating layer 52 and/or between the waveguide 16 and theheater 51. At least one of the claddings 54 can directly contact thelight-transmitting medium 18. A cladding that contactslight-transmitting medium 18 preferably has a lower index of refractionthan the light-transmitting medium 18. When the light-transmittingmedium 18 is silicon, suitable claddings include, but are not limitedto, polymers, silica, SiN and LiNbO₃. In some instances, a single layerof material can serve as both a cladding 54 and an insulating layer 52.Although the insulating layer 52 is shown as a single layer of material,the insulating layer 52 can include or consist of multiple layers ofmaterial.

When one or more layers of material are positioned over the ridge 22,the one of more layers of material define a device ridge. For instance,the perimeter of the insulating layer 52 illustrated in FIG. 6B definesthe perimeter of a device ridge 55. The device ridge includes the ridge22 in that at least a portion of the ridge 22 is positioned within thedevice ridge. When one or more material layers are not positioned overthe ridge 22, the ridge 22 serves as the device ridge 55. For instance,if the cladding 54 and insulating layer 52 were not present on thedevice of FIG. 6B, the ridge 22 would serve as the device ridge 55.

Conductors 56 are in electrical communication with the heater and arepositioned so as to provide electrical communication between the heater51 and contact pads 58. The conductors 56 and contact pads 58 can beelectrically conducting. The conductors 56 can include a contact region62 in electrical communication with the heater. As a result, theelectronics 47 can apply electrical energy to the contact pads 58 so asto deliver electrical energy to the heater 51 and can accordinglyoperate the heater so the heater 51 generates heat. The location of theheater over the lateral side of the ridge 22 allows the generated heatto elevate the temperature of the ridge through a mechanism such asconduction.

In some instances, the heater 51 is an “electrical resistance heater.”For instance, the heater 51 can include or consist of an electricallyconducting layer 60 that serves as a resistor. The length of the ridge22 that is heated by the heater can be changed by changing the length ofthe resistor. An example of a suitable resistor is a trace that includesor consists of a metal, metal alloy. Examples heaters include or consistof titanium traces, tungsten traces, titanium nitride, and nichrometraces. The contact regions 62 can be positioned over or under theconducting layer 60 and in contact with a contact portion of theconducting layer 60. During operation of the device, the electronics 47can drive sufficient electrical current through the electricallyconducting layer 60 to cause the electrically conducting layer 60 togenerate the heat that is conducted to the modulator.

The electrically conducting layer 60 can have a higher resistance/lengththan the contact regions 62 and/or the conductors 56 in order to stop orreduce generation of heat by the conductors 56. This can be achieved byusing different materials and/or dimensions for the conductors 56 andthe conducting layer 60. For instance, the conductors 56 can be aluminumwhile the conducting layer 60 that serves as the heater is titanium.Titanium has a specific electrical resistance of about 55 μohm-cm whilealuminum has a specific electrical resistance of about 2.7 μohm-cm. As aresult, the conductors 56 and conducting layer 60 can have similar crosssectional dimensions and an electrical current can be driven through theconductors 56 and conducting layer such that heat is generated at theconducting layer without undesirable levels of heat being generated bythe conductors 56. Alternately, the conductors 56 can have larger crosssection dimensions than the heater in order to further reduce heatgeneration by the conductors 56

In some instances, the conductors 56 include a portion of the conductinglayer 60 from the heater 51 in addition to the contact regions 62 as isevident in FIG. 6A. In these instances, the contact regions 62 can bemore conductive and/or have larger dimensions than the conducting layer60 in order to reduce generation of heat by the conductor 56. When theconductors 56 include the conducting layer 60 and the contact regions62, the conductors 56 and heater can be formed by forming a first layerof the material for the conducting layer 60 and then forming a secondlayer of material for the conductors 56 over the first layer. Suitablemethods for forming the first layer and the second layer on the deviceinclude, but are not limited to, sputtering, evaporation, PECVD andLPCVD. The first layer and the second layer can then be patterned so asto form the conductors 56 and conducting layer 60 on the device.Suitable methods for patterning include, but are not limited to, etchingin the presence of one or more masks. The portion of the second layerover the heater 51 can then be removed to provide the configuration ofconducting layer and conductive layer shown in FIG. 6A and FIG. 6C.Suitable methods for removing the portion of the second layer include,but are not limited to, etching in the presence of a mask. Although theelectrically conducting layer 60 and conductors 56 are disclosed as asingle layer of material, either or both of the conducting layer 60 andthe conductors 56 can include or consist of one or more different layersof material.

FIG. 6A through FIG. 6C illustrate the heater 51 as being positionedover a lateral side of the ridge without any portion of the heater beingpositioned above the upper exterior cornet of the device ridge. However,the heater can extend above an upper exterior corner of the device ridgewithout being position over the device ridge or can bend around theexterior corner of the device ridge so as to be positioned over the topof the ridge 22. For instance, FIG. 7A is a cross section of a devicehaving a heater that bends around an exterior corner of the device ridgesuch that a portion of the heater is positioned over the top of theridge 22. The heater can be positioned over the device ridge withoutbeing positioned over the modulator or can extend over the modulator.For instance, the heater can be positioned over the device ridge withoutextending over the ridge 22 of electro-absorption medium or can bepositioned over the over the ridge 22 of electro-absorption medium. Asdiscussed above, having a significant portion of the heater over the topof the ridge can pull the mode upwards in the ridge. As a result, it maybe desirable to eliminate the portion of the heater from over the top ofthe ridge. Further, the portion of the heater over the top of the ridgemay be an artifact of the fabrication process.

FIG. 7A illustrates the heater extending from an upper exterior cornerof the device ridge to a lower interior corner at the base of the deviceridge; however, the heater can be positioned over the lateral sidewithout extending from an upper exterior corner to a lower interiorcorner. For instance, the heater 51 can be spaced apart from the upperexterior corner and/or lower interior corner.

In some instances, a portion of the heater extends away from the ridge22 such that the heater 51 is positioned over the slab regions. Forinstance, FIG. 7B is a cross section of the modulator where the heaterextends down to the lower interior corned of the device ridge and alsoextends away from the lower interior corner over a slab region. Theheater has a first portion positioned over a lateral side of the ridgeand a second portion that extends away from the first portion and ispositioned over a slab region. The second portion can be formed suchthat an imaginary line can be drawn that is both perpendicular to theslab region and extending through the second portion without extendingthrough the first portion. Additionally or alternately, an imaginaryline can be drawn perpendicular to a lateral side of the ridge andextending through the first portion without extending through the secondportion. The distance that the second portion of the heater extends awayfrom the first portion of the heater is labeled E in FIG. 7B. Thedistance is equal to the distance between the edge of the heater and theportion of the heater on the lateral side of the ridge 22. Increasingthe distance that the heater extends away from the device ridge canreduce the degree of localized heating and can increase the powerrequirements for the device. As a result, it is often desirable tominimize or eliminate the portion of the heater that extends away fromthe device ridge. Further, the portion of the heater that extends awayfrom the device ridge may be an artifact of the fabrication process. Insome instances, the distance that the heater extends away from thedevice ridge is 0 or is less than 2 μm, or 0.5 μm and can be 0 μm.

The heater configured to FIG. 7A and FIG. 7B can be combined. Forinstance, a portion of a heater 51 can be positioned over the top of theridge 22 as disclosed in the context of FIG. 7A while another portion ofthe heater extends away from the ridge 22 as disclosed in the context ofFIG. 7B. FIG. 7C illustrates an example of a device having a heaterwhere a portion of the heater 51 is positioned over the top of the ridge22 while another portion of the heater extends away from the ridge 22.

Although FIG. 6A through FIG. 7C illustrate the device having a singleheater, the device can include multiple heaters. As an example, FIG. 8Aand FIG. 8B illustrate a device having heaters over the lateral sides ofthe ridge. FIG. 8A is a topview of the portion of the device thatincludes a modulator. FIG. 8B is a cross section of the device shown inFIG. 8A taken along the line labeled B in FIG. 8A.

The heaters are positioned on opposing lateral sides of the ridge. Forinstance, a cross section taken perpendicular to the longitudinal axisof the waveguide extends through both of the heaters. Since the heatersare on opposing laterals sides, thermal energy from different heaterscan heat the same region of the waveguide. The use of both heaters canfurther increase temperature uniformity within the waveguide but mayform parasitic capacitance that can reduce RF performance and operationspeed.

As is evident from FIG. 8A, each heater is in electrical communicationwith different contact pads 58. As a result, the electronics canindependently operate the heaters. Alternately, the electronics canconnect the heaters in parallel and/or series. Alternately, the devicecan include traces or other conductors that connect the heaters inparallel and/or series.

Although the heaters shown in FIG. 8A through FIG. 8B are constructedaccording to FIG. 6B, each of the heaters can be constructed accordingto any one of FIG. 6B and FIG. 7A through FIG. 7D. Additionally, theheaters need not have the same construction. For instance, one of theheaters can be constructed according to FIG. 7C while another heater isconstructed according to FIG. 7A.

The portion of the heater vertically over the top of the device ridge inFIG. 7A is labeled OA in FIG. 7A. For instance, the portion of theheater labeled OA represents the portion of the heater that overlaps themodulator in FIG. 7A or that would be vertically projected onto the topof the modulator. The portion of a heater located over the ridge can bean artifact of the fabrication process. The width of the top of themodulator is labeled W in FIG. 7A. In some instances, W is less than 1.6μm, 1.4 μm, or 1.2 μm and/or greater than 0.8 μm, 0.6 μm, or 0.4 μm.Since it can be desirable to reduce or eliminate the portion of a heaterlocated over the top of the modulator, the percentage of the width ofthe top of the modulator covered by the heater (OA/W) can be 0 or lessthan less than 5%, 10%, or 15% at a particular location along the lengthof the modulator for a portion of the modulator length or the entiremodulator length. When the device includes two heaters on opposinglaterals sides of the ridge and one or more of the heaters is positionedover the modulator, the percentage of the width covered by the heaterscan be 0 or less than 10%, 20%, or 30% at a particular location alongthe length of the modulator for a portion of the modulator length or theentire modulator length. Although the above overlap dimensions aredisclosed in the context of a heater constructed according to FIG. 7A,these dimensions can apply to a heater constructed according to FIG. 7Cand any other heater where a portion of the heater is located over alateral side of the ridge.

The portion of the heater over a lateral side of the modulator in FIG.6B is labeled OP. For instance, the portion of the heater labeled OPrepresents the portion of the heater that overlaps the lateral sidemodulator in FIG. 6B or that would be horizontally projected onto thelateral side of the modulator. The height of the lateral side of themodulator is labeled H in FIG. 6B. In some instances, the waveguideheight is more than 0 μm, 2 μm, or 3 μm and/or less than 4 μm, 5 μm, or6 μm. In some instances, the presence of an insulating layer 52 and/orcladding 54 between a slab region and the heater prevent the heater frombeing positioned over the entire height of the lateral side as isevident in FIG. 6B. However, it may be desirable to increase the portionof lateral side height that is covered by the heater in order toincrease temperature uniformity. Accordingly, the heater can cover theportion of the lateral side extending from the upper corner of the ridge22 to the base of the ridge 22, the base of the insulating layer 52 orthe base of the cladding 54. In some instances, the percentage of thelateral side covered by the heater (OP/H) can be greater than 10%, or 5%and/or less than or equal to 90%, 100% at a particular location alongthe length of the modulator for a portion of the modulator length or theentire modulator length. These dimensions can be met with a heater thatextends from an exterior corner to in interior corner or with a heaterthat is spaced apart from the interior corner and/or exterior corner.Although the above overlap dimensions are disclosed in the context of aheater constructed according to FIG. 6B, these dimensions can apply to aheater constructed according to FIG. 7A through FIG. 7C and any otherheater where a portion of the heater is located over a lateral side ofthe ridge.

The waveguide height is labeled h in FIG. 6B. In some instances, thewaveguide height is more than 1 μm, 2 μm, or 3 μm and/or less than 4 μm,5 μm, or 6 μm. The height of the lateral side can be more than 70%,80%%, or 95%% and/or less than 97%%, 98%%, or 99% of the waveguideheight. Additionally or alternately, the heater can be positioned overmore than 70%, 75%, or 80% and/or less than 90%, 95%, or 100% of thewaveguide height at a particular location along the length of themodulator for a portion of the modulator length or the entire modulatorlength.

Moving the heater 51 closer to the ridge 22 reduces the distance overwhich the generated heat must be conducted in order to elevate thetemperature of the modulator and can accordingly reduce the amount ofheat that must be generated in order to achieve a particular temperaturewithin the modulator. Reducing the thickness of the one or more layersof material between the heater and the ridge 22 can move the heater 51closer to the ridge 22 or electro-absorption medium. For instance,reducing the thickness of the one or more claddings 54 and the one ormore insulating layers 52 can move the heater 51 closer to the ridge 22or electro-absorption medium. In some instances, all or a portion of theheater 51 is within 0.5 or 2 μm of the electro-absorption medium 27. Insome instances, the heater 51 is arranged such that the heater 51 doesnot contact the device at a location that is more than 2 μm, 200 μm, or500 μm away from the ridge 22.

The details of the modulator construction are not illustrated in FIG. 6Athrough FIG. 8B; however, the modulator can have a variety ofconstructions including, but not limited to, the constructions of FIG.2E through FIG. 4C. In order to illustrate this concept, FIG. 9A andFIG. 9B illustrate the device of FIG. 6A through FIG. 6C in combinationwith the modulator of FIG. 2E. FIG. 9A is a topview of the device. FIG.9B is a cross section of the device shown in FIG. 9A taken along theline labeled B in FIG. 9A. The insulating layer 52 and cladding 54 arenot shown in FIG. 9A in order to show the underlying features. FIG. 9Ashows the heater 51 extending beyond the perimeter of theelectro-absorption medium 27; however, one or both ends of theelectro-absorption medium 27 can terminate over the electro-absorptionmedium 27.

As is evident in FIG. 9B, a protective layer 64 can optionally be formedover the above devices. In some instances, the protective layer 64 canhave a thermal conductivity that is less than the thermal conductivityof the one or more claddings 54 and/or the one or more insulating layers52. The reduced thermal conductivity of the protective layer 64 causesheat generated by the heater to be directed toward the modulator and canaccordingly reduce the energy requirements of the heater as well asreduce thermal cross talk. Suitable protective layers include, but areno limited to, silica, silicon nitride, and aluminum oxide. Although theprotective layer is disclosed as a single layer of material, theprotective layer can be constructed of multiple layers of material. Insome instances, one, two or three layers of the protective layer have athermal conductivity greater than 0.75 WK/m, 1.0 WK/m, or 1.25 WK/m. Theprotective layer is not illustrated in FIG. 8B.

The above devices can optionally include one or more thermal conductorspositioned so as to conduct thermal energy away from the ridge. The oneor more thermal conductor can increase the uniformity of the temperaturedistribution along the ridge. The increased temperature uniformityallows higher power levels to be used with the modulator. FIG. 10A andFIG. 10B illustrate the device of FIG. 9A and FIG. 9B modified so as toinclude thermal conductors 68. Accordingly, the device is illustratedusing the modulator of FIG. 2E as an example. FIG. 10A is a topview ofthe device. FIG. 10B is a cross section of the device shown in FIG. 10Ataken along the line labeled B in FIG. 10A. The insulating layer 52 andcladding 54 are not shown in FIG. 10A in order to show the underlyingfeatures.

Conductors 68 are each positioned on a slab region. In the illustratedembodiment, at least a portion of each conductor is positioned on theslab regions of the electro-absorption medium 27. In FIG. 10A dashedlines are used to represent the perimeter of the electro-absorptionmedium 27 under the conductors 68 and also to illustrate the perimeterof the recesses 25 under the conductors 68. The conductors can bethermally conductive and, in some instances, are also electricallyconducting.

In the illustrated example, the conductors 68 extend from over theelectro-absorption medium 27 to a position over the light-transmittingmedium 18. For instance, a first portion of each conductor 68 ispositioned such that the electro-absorption medium 27 is between theconductor 68 and the base 20 and a second portion of the conductor 68 ispositioned such that the electro-absorption medium 27 is not between theconductor 68 and the base 20. In some instances, the first portion ofthe conductor 68 is arranged such that a line can be drawn that isperpendicular to a surface of the conductor 68 and also extends throughthe electro-absorption medium 27 and in the second portion of theconductor 68, a line can be drawn that is perpendicular to same surfaceof the conductor 68 while extending through the light-transmittingmedium 18 but not through the electro-absorption medium 27. Although aportion of a conductor 68 can be positioned in a recess 25, in someinstances, at least a portion of the conductor 68 is located outside ofthe recess as is evident from FIG. 10A and FIG. 10B.

The one or more claddings 54 are optionally positioned between theconductors 68 and the base 20. In some instances, a cladding 54 isbetween one of the conductors 68 and a doped region 40. The claddings 54can directly contact the light-transmitting medium 18. The insulatinglayer 52 can be located over the conductors 68. In some instances, asingle layer of material can serve as both a cladding 54 and aninsulating layer 52. Although the insulating layer 52 is shown as asingle layer of material, the insulating layer 52 can include or consistof multiple layers of material.

The conductors 68 can be in electrical communication with a dopedregion. For instance, in some instances, the conductors are in directphysical contact with doped region 40. As an example, the conductors canextend through an opening in the material layer into contact with theunderlying doped region as shown in FIG. 10B. The conductors can also bein electrical communication with contact pads 70. For instance, openingsin the one or more layers of material can expose one or more portions ofthe conductors 68 that act as contact pads 70. The contact pads 70 canbe in electrical communication with the electronics. The contact pads 70can serve as the electrical conductors 44 disclosed in FIG. 2E, etc.Accordingly, when the conductors 68 are electrically conducting, one ormore of the conductors 68 can provide electrical communication betweenthe electronics and a doped region 40. In some instances, the conductors68 are electrically conducting and the doped regions 40 are doped at aconcentration that allows the doped regions 40 to act as electricalconductors. Accordingly, the conductors 68 and doped regions 40 providean electrical pathway between the electronics and the ridge ofelectro-absorption medium 27. As a result, the electronics can applyenergy to the electrical conductors 68 in order to apply an electricfield to the electro-absorption medium 27 and operate the modulator.

During operation of the modulator, heat is generated as a result of theelectro-absorption medium 27 absorbing light during the operation of themodulator. The label of “light direction” is used in FIG. 10A toindicate that the direction of propagation for light signals duringoperation of the modulator. The light signal enters theelectro-absorption medium 27 through an input side of theelectro-absorption medium 27 and exits from the electro-absorptionmedium 27 through an output side of the electro-absorption medium 27.Generally, the light absorption is most intense where the light signalfirst interacts with the electrical field. In general, this occurs atthe interface of the light-transmitting medium 18 and theelectro-absorption medium 27. As a result, light absorption is generallymost intense at or near the input side of the electro-absorption medium27. The increased light absorption can lead to a hot spot in themodulator. When the conductors are thermal conductors, the extension ofthe conductors 68 from over the electro-absorption medium 27, across theinput side of the electro-absorption medium 27 to a location over thelight-transmitting medium 18 provides a pathway for the heat generatedby the modulator to be carried away from the modulator and accordinglyprovides cooling of the modulator.

The conductors 86 include an active edge 72 that is the edge of theconductors 86 located closest to the ridge 22. The modulation of thelight signal primarily occurs where the electrical field is formed inthe electro-absorption medium 27. Accordingly, the doped regions 40define an entry side 74 and an exit side 76 for the modulator. Theconductors 86 are constructed such that when moving from the entry side74 to the exit side 76 for at least a portion of the conductor, theactive edge 72 moves away from the ridge of the electro-absorptionmedium. More specifically, the distance between the active edge 72 andthe ridge 22 increases moving from the entry side 74 to the exit side 76for at least a portion of the active edge 72 where the distance ismeasured perpendicular to a lateral side of the ridge 22. As noted, heatis generated as a result of the electro-absorption medium 27 absorbinglight during the operation of the modulator and is generally mostintense where the light signal first interacts with the electricalfield. As a result, the heat is generally generated most intensely nearthe entry side 74. The active edge 72 being closer to the ridge 22 atthe entry side 74 results in a more efficient conduction of heat awayfrom the ridge 22 near the entry side 74 than occurs further downstreamof the entry side. The increased efficiency of heat conduction near theentry side 74 reduces the formation of hot spots at or near the entryside 74 of the modulator. The active edge 72 becoming further from theridge downstream of the entry side results in a less efficientconduction of heat away from the ridge at locations downstream of theentry side 74. As a result, the heat provided by the heater 51 remainsin the ridge 22 longer at locations downstream of the entry side 74where less heat is generated as a result of absorption. By conductingheat away from the location where heat is generated by absorption butallowing heat from the heater to remain in the waveguide at locationswhere less heat is generated by absorption, the uniformity of thetemperature along the length of modulator is increased. Increasing thisuniformity allows the modulator to be operated at an efficienttemperature.

In FIG. 10A, the active edge 72 of the conductors 86 extends past theentry side 74 of the modulator; however, the active edge 72 of theconductors 86 can terminate between the entry side 74 and the exit side76. Extending the active edge 72 of the thermal conductors past theentry side 74 increases the area of the conductor 43 that is availablefor carrying heat away from the entry side 74 and may be the moredesirable arrangement. Although FIG. 10A illustrates the active edge 72terminating between the entry side 74 and the exit side 76 withoutextending past the exit side 76, the active edge 72 of the conductors 86can extend past the exit side 76.

One or more of the conductors 86 can extend over the output side of theelectro-absorption medium 27 and/or over the input side of theelectro-absorption medium 27. This arrangement can increase theefficiency at which heat is conducted away from the entry side of themodulator.

The active edge 72 of each conductor illustrated in FIG. 10A includestwo portions that are parallel or substantially parallel to the ridge 22connected by a connecting portion; however, the active edge 72 can haveother configurations constructed such that when moving from the entryside 74 to the exit side 76 for at least a portion of the conductor, theactive edge 72 moves away from the ridge of the electro-absorptionmedium. For instance, the active edge can have a stair stepconfiguration, one or more curved regions, and/or other configurations.More specifically, the distance between the active edge 72 and the ridge22 can gradually increases moving from the entry side to the exit sidewhere the distance is measured perpendicular to a lateral side of theridge 22.

When the conductors 68 provides heat dissipation, the second portion ofthe conductors can have dimensions that exceed the dimensions that arecommonly used for metal traces designed to carry electrical currents onintegrated circuit boards. For instance, one or more of the conductors68 can cover a portion of the device having an area more than 4,000microns², 20,000 microns², or 100,000 microns² and/or a perimeter lengthgreater than 300 microns, 500 microns, or 1000 microns. Additionally oralternately, one or more of the conductors can have a thickness greaterthan 50 picometers, 0.1 micron, or 1 micron. In some instances, the oneor more of the conductors a thickness greater than 50 picometers, 0.1micron, or 1 micron for more than 50% or 85% of the device area coveredby the conductor. Although FIG. 10A illustrates the conductors 68 ashaving a somewhat regular shape, one or more of the conductors can haveirregular shapes and/or patterns.

Although FIG. 10A and FIG. 10B illustrates the conductors 68 terminatingbefore contacting the ridge, a portion of each one of the one or moreconductors 68 can extend into contact with the ridge or with a verticalportion of the cladding 54.

Although the conductors 68 are disclosed as serving as the electricalconductors 44, the conductors need not serve as the electricalconductors 44. As a result, the conductors 68 need not be in electricalcommunication with the doped regions of the modulator.

Additional information regarding the use and construction of conductors68 can be found in U.S. patent application Ser. No. 14/670,292; filed onMar. 26, 2015; entitled “Control of Thermal Energy in Optical Devices;”and incorporated herein in its entirety.

The waveguide in the above illustrations include a modulator portionbetween a first portion of the waveguide and a second portion of thewaveguide. The above illustrations show a straight intersection betweenthe first portion and the modulator portion and also between the secondportion and the modulator portion. However, the waveguide portions inany of the above devices can intersect an angle other than 180° at oneor both of these intersections. For instance, FIG. 11 shows the deviceof FIG. 2A modified such that an angle θ between a modulator portion 87of the waveguide 16 and a first portion 88 of the waveguide 16 is lessthan 180° and an angle θ between the modulator portion 87 of thewaveguide 16 and a second portion 89 of the waveguide 16 is less than180°. The angle θ can be the same or different from the angle ϕ. Theparticular angle between the different portions of the waveguide can beselected to reduce optical loss from optical effects such as refraction.In some instances, the angle at one or both of these intersections isgreater than 120°, 150°, 160°, or 170° and/or less than 175°, or 180°.

The modulators included in the devices of FIG. 6A through FIG. 11 canhave constructions other than the constructions of FIG. 1A through FIG.4C. For instance, the doped regions 40 disclosed above can be replacedby other filed sources such as metal electrical conductors. Additionallyor alternately, the slab regions of the electro-absorption medium 27 canbe removed by a process such as etching. Examples of other suitablemodulator constructions can be found in U.S. patent application Ser. No.12/653,547, filed on Dec. 15, 2009, entitled “Optical Device HavingModulator Employing Horizontal Electrical Field,” and U.S. patentapplication Ser. No. 13/385,774, filed on Mar. 4, 2012, entitled“Integration of Components on Optical Device,” each of which isincorporated herein in its entirety. U.S. patent application Ser. Nos.12/653,547 and 13/385,774 also provide additional details about thefabrication, structure and operation of these modulators. In someinstances, the modulator is constructed and operated as shown in U.S.patent application Ser. No. 11/146,898; filed on Jun. 7, 2005; entitled“High Speed Optical Phase Modulator,” and now U.S. Pat. No. 7,394,948;or as disclosed in U.S. patent application Ser. No. 11/147,403; filed onJun. 7, 2005; entitled “High Speed Optical Intensity Modulator,” and nowU.S. Pat. No. 7,394,949; or as disclosed in U.S. patent application Ser.No. 12/154,435; filed on May 21, 2008; entitled “High Speed OpticalPhase Modulator,” and now U.S. Pat. No. 7,652,630; or as disclosed inU.S. patent application Ser. No. 12/319,718; filed on Jan. 8, 2009; andentitled “High Speed Optical Modulator;” or as disclosed in U.S. patentapplication Ser. No. 12/928,076; filed on Dec. 1, 2010; and entitled“Ring Resonator with Wavelength Selectivity;” or as disclosed in U.S.patent application Ser. No. 12/228,671, filed on Aug. 13, 2008, andentitled “Electrooptic Silicon Modulator with Enhanced Bandwidth;” or asdisclosed in U.S. patent application Ser. No. 12/660,149, filed on Feb.19, 2010, and entitled “Reducing Optical Loss in Optical Modulator UsingDepletion Region;” each of which is incorporated herein in its entirety.

The heater 51, one or more insulating layers 52, one or more claddings54, and conductors 56 can be fabricated using fabrication technologiesthat are employed in the fabrication of integrated circuits,optoelectronic circuits, and/or optical devices. FIG. 12A through FIG.12F illustrate a variety of methods for fabricating the heater. Themethods can be performed on a wafer such as a silicon-on-insulatorwafer. A modulator can be formed on the wafer so as to provide a deviceprecursor having the cross section illustrated in FIG. 12A. The one ormore claddings 54 and one or more insulating layers 52 are formed overthe modulator. A heater precursor 90 is formed over the one or moreinsulating layers 52. The heater precursor can be a layer of materialthat is the same as the material that is desired for the conductinglayer 60 that serves as a resistive heater. For instance, if a titaniumtrace is to serve as a resistive heater, the heater precursor 90 can bea layer of titanium. A first mask 92 is formed so as to protect theregion of the heater precursor 90 where the heater is to be formed. Asuitable first mask includes, but is not limited to, a photoresist,silica, and silicon nitride. Photoresist formation techniques willgenerally form a photoresist that is wider than the thickness of theheater precursor 90. As a result, in some instances, the first mask willbe positioned over the device ridge as shown in FIG. 12A.

A first etch is performed on the device precursor of FIG. 12A so as toprovide the device precursor of FIG. 12B. A suitable first etchincludes, but is not limited to, an anisotropic etch and/or a dry etch.When the etch direction is vertical, a portion of the heater precursor90 that is not protected by the first mask 92 remains in place on thedevice ridge as shown in FIG. 12B.

A second etch can be performed on the device precursor of FIG. 12B so asto provide the device precursor of FIG. 12C. A suitable second etchincludes, but is not limited to, an isotropic and/or a wet etch thatremoves the portion of the heater precursor 90 that is not protected bythe first mask 92. Additionally, an isotropic and/or a wet etchundercuts the first mask as shown in FIG. 12C. The second etch can beperformed for a duration that is sufficient for the second etch toundercut the first mask 92 until the portion of the heater precursor 90located over the device ridge is removed.

The first mask 92 can be removed from the device precursor of FIG. 12Cso as to provide the device of FIG. 12D. The portion of the heaterprecursor 90 that remains on the device serves as the heater 51. Asshown in FIG. 12D, it is possible for the heater to extend past a cornerof the device ridge without being positioned over the top of the deviceridge.

An alternative method can start with the device precursor of FIG. 12B.For instance, the first mask 92 of FIG. 12B can be removed and a secondmask formed on the resulting device precursor so as to provide thedevice precursor of FIG. 12E. The second mask 94 protects the sameportion of the heater precursor 90 that was protected by the first mask92 but the second mask 94 protects the edges of the heater precursor 90rather than protecting only the top of the heater precursor 90. Asuitable second mask 94 includes, but is not limited to, a photoresist,silica, and silicon nitride.

A third etch can be performed on the device precursor of FIG. 12E andthe second mask 94 removed so as to provide constructed according toprecursor of FIG. 7C. A suitable third etch includes, but is not limitedto, an isotropic and/or a wet etch that removes the portion of theheater precursor 90 that is not protected by the second mask 94. Theedges of the resulting heater are well-defined, however, the method canresult in a portion of the heater being located over the device ridgeand/or extending away from the base of the device ridge.

The methods of FIG. 12A through FIG. 12E are easily adapted to formheaters on opposing sides of the ridge. For instance, heaters onopposing sides of the ridge and constructed according to FIG. 6B can begenerated by performing the first etch without the first mask.Alternately, heaters constructed according to FIG. 12D can be generatedby forming the first mask formed so as to protect each location whereone of the heaters will be formed and performing the first etch followedby the second etch. Alternately, heaters can be generated on opposinglateral sides of the by forming the first mask formed so as to protecteach location where one of the heaters will be formed and performing thefirst etch. The first mask can be removed and the second mask can beformed over the remaining heater precursor as described above. The thirdetch can then be performed and the second mask removed so as to provideheaters constructed according to FIG. 7C.

The device can include one or more temperature sensors (not shown) thatare each positioned to sense the temperature of the modulator and/or thetemperature of a zone adjacent to the modulator. Suitable temperaturesensors include, but are not limited to, thermocouples, thermistors,integrated PN diodes, or other integrated semiconductor devices. Theelectronics can adjust the level of electrical energy applied to theheater in response to the output received from the one or moretemperature sensors in a feedback loop. For instance, the electronicscan operate the heater such that the temperature of the heater stays ator above a threshold temperature (T_(th)) during operation of thedevice. For instance, when the electronics determine that thetemperature of the modulator falls below the threshold temperature, theelectronics can apply electrical energy to the heater so as to bring thetemperature of the modulator to or above the threshold temperature.However, when the electronics determine that the temperature of themodulator falls above the threshold temperature, the electronics canrefrain from applying the electrical energy to the heater. As a result,when the electronics determine that the temperature of the modulator isabove the threshold temperature, the temperature of the modulator canfloat in response to the operation of the device in the ambientatmosphere.

Although the heater is disclosed as being positioned on the ridge of amodulator, the heater can be positioned on the ridge of other opticalcomponents such as light sensors and light sources such as are disclosedin U.S. patent application Ser. No. 13/506,629.

Example 1

A Quantum-Confined Stark Effect (QCSE) modulator was constructedaccording to FIG. 4A and FIG. 4A and FIG. 4C. An electro-absorptionmedium of Si_(0.26)Ge_(0.74) was epitaxially grown to a thickness of1150 nm on the silicon light-transmitting medium of asilicon-on-insulator wafer in a chamber at a pressure of 50 Torr and achamber temperature of 500° C. The upper surface of the resultingSi_(0.26)Ge_(0.74) had a root mean square roughness of about 1 nm. TheSi_(0.26)Ge_(0.74) was used as a seed layer for epitaxial growth of aSi_(0.26)Ge_(0.74) potential barriers. Quantum wells of Ge and potentialbarriers of Si_(0.4)Ge_(0.6) were grown on the result in an alternatingpattern so as to form a quantum well structure with 10 layers that eachincluded four sub-layers. The wells were grown to a thickness of 9 nmand the potential barriers were grown to a thickness of 12 nm. Theuppermost layer of the quantum well structure was a potential barrier ofSi_(0.4)Ge_(0.6). A layer of Si_(0.26)Ge_(0.74) was grown to a thicknessof 1328 nm on the quantum well structure to serve as the upper layer ofelectro-absorption medium 27. The upper surface of the upper layer ofSi_(0.26)Ge_(0.74) had a root mean square roughness of about 2 nm. Theresult was annealed in three annealing periods separated by restperiods. During the annealing periods, the quantum well structure wasexposed to a temperature of 900° C. for 15 minutes. During the restperiods, the quantum well structure was exposed to ultra-high vacuum andtemperature of 500° C. for 1 minute. The resulting modulator had aworking wavelength shift rate of 0.83 nm/° C.

Other embodiments, combinations and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawings.

The invention claimed is:
 1. An optical device, comprising: a waveguidepositioned on a base and a modulator positioned on the base, themodulator including a ridge that includes Si_(1-x)Ge_(x) where x isgreater than or equal to 0.4 and less than or equal to 0.8, themodulator configured to guide a light signal through the modulator suchthat the light signal contacts the Si_(1-x)Ge_(x); and a device ridgethat includes the ridge, the device ridge having a top side and lateralsides, the lateral sides being between the top side and the base, andthe top side having a width; and a heater positioned over one of thelateral sides and the top side of the device ridge without beingpositioned over the entire width of the top side.
 2. The device of claim1, wherein the modulator is a Quantum-Confined Stark Effect (QCSE)electro-absorption modulator configured to use the Quantum-ConfinedStark Effect to modulate light signals.
 3. The device of claim 1,wherein the ridge includes a quantum well structure that includes theSi_(1-x)Ge_(x).
 4. The device of claim 3, wherein the quantum wellstructure includes potential barriers alternated with quantum wells. 5.The device of claim 4, wherein the potential barriers include theSi_(1-x)Ge_(x).
 6. The device of claim 5, wherein the quantum wells areGe_(y)Si_(x) where 0.9≤y≤1.0 and 0.1≤x≤0.0.
 7. The device of claim 6,wherein a height of the modulator is less than 5 μm.
 8. The device ofclaim 3, wherein the quantum well structure includes multiple layersthat each have the same chemical composition.
 9. The device of claim 3,wherein the quantum well structure contacts a seed layer and a root meansquare roughness of a surface between the quantum well structure and theseed layer is less than 1 nm.
 10. An optical device, comprising: awaveguide positioned on a base and a modulator positioned on the base,the modulator being a Quantum-Confined Stark Effect (QCSE)electro-absorption modulator and including a ridge that includesSi_(1-x)Ge_(x) where x is greater than or equal to 0.4 and less than orequal to 0.8, the modulator configured to guide a light signal throughthe modulator such that the light signal contacts the Si_(1-x)Ge_(x).11. The device of claim 10, wherein the ridge includes a quantum wellstructure that includes the Si_(1-x)Ge_(x).
 12. The device of claim 11,wherein the quantum well structure includes potential barriersalternated with quantum wells.
 13. The device of claim 12, wherein thepotential barriers include the Si_(1-x)Ge_(x).
 14. The device of claim13, wherein the quantum wells are Ge_(y)Si_(x) where 0.9≤y≤1.0 and0.1≤x≤0.0.
 15. The device of claim 10, wherein a height of the modulatoris less than 5 μm.
 16. The device of claim 10, wherein the light signalhas a wavelength of in a range of 1250 nm to 1350 nm.
 17. A method offabricating a modulator, comprising: growing a seed layer on a base suchthat a surface of the seed layer has a root mean square roughness lessthan 1 nm; and growing a quantum well structure on the surface of theseed layer, the quantum well structure including Si_(1-x)Ge_(x) where xis greater than or equal to 0.4 and less than or equal to 0.8.
 18. Themethod of claim 17, wherein a portion of the quantum well structure thatcontacts the surface is epitaxially grown in a chamber at a pressure of20 to 60 Torr and at a temperature of 500 to 900 degrees.
 19. The methodof claim 17, wherein the quantum well structure alternates wells withpotential barriers, the wells being Ge and the potential barriersincluding the Si_(1-x)Ge_(x).